FIG. 1 is a block diagram which shows a conventional NT1 (network termination equipment).
The equipment comprises a transmitter circuit 11 and a receiver circuit 12 on a terminal equipment side, a receiver circuit 13 and a transmitter circuit 14 on a subscriber line side, a circuit termination 15, a call origination signal detector 16, a call origination signal transmitter 17, a power feeder 18 and a power unit 19.
FIG. 2 is a sequence chart to show operations when the equipment is activated.
When an activation request for NT1 is made at a terminal equipment (TE), the equipment transmits a call origination signal to the NT1. The call origination signal detector 16 in the NT1 detects the signal and feeds a detection output to the call origination signal transmitter 17. The transmitter 17 transmits a call origination signal to the line termination equipment at the telephone office via a subscriber line. The call origination may be notified by closing the loop on a subscriber line or by transmitting a tone signal.
The line termination equipment at the telephone office originates an activation request for a subscriber line when it receives a call origination signal, and transmits training pulses to the NT1. The receiver 13 receives the training pulses and conducts adaptive line equalization when it receives them. When equalization ends and when frame synchronization is established by the circuit 15, the transmitter 14 transmits training pulses to the equipment at the telephone office. When adaptive line equalization is completed at the office equipment and the frame synchronization is established, the activation of subscriber line is completed to start communication.
FIG. 2 shows the case of activation by an incoming call of a caller. FIG. 3 shows the activation sequence at the time of a loopback test.
The line termination equipment at the telephone office transmits training pulses to the NT1 upon request of activation for a subscriber line, and the NT1 in turn transmits training pulses to the equipment at the office after having established frame synchronization.
The equipment at the office transmits a control signal for loopback test to the NT1 by using control bits within the frame structure, on the subscriber line after having established the frame synchronization.
When the circuit termination 15 detects a control signal indicative of loopback test, it transmits an SW1 driving signal to switch SW1 for the loopback to close the loop by turning on the switch SW1. Then, the circuit 15 sends to the equipment at the office an indication signal of loopback test by using control bits within the frame structure on the subscriber line.
The equipment at the office recognizes completion of loop closure for loopback at the NT1 when it receives the indication signal, and measures a bit error rate on the loopback path.
As stated above, the conventional NT1 closes a loop under control of the equipment at the central office, to test the loopback path. Such a test verifies the operations of the transmitting/receiving lines, both at the subscriber line side and at the terminal equipment side of the NT1. It also tests the circuit terminations, and the power unit which supplies power to the above circuits.
These operations are controlled by the circuit termination 15. In order to facilitate the understanding, description will now be given of the operation of the circuit 15.
The circuit termination 15 has four modes of operation, an initial mode, an activation mode, a communication mode, and a loopback test mode. FIG. 4 shows the relation among these modes, and FIGS. 5 through 7 show respectively the operation flows of the above four modes.
In the initial mode (FIG. 5), the circuit termination 15 does not send out signals to the circuits 11 and 14, but rather waits for training pulses from the line termination equipment at the telephone office. When the training pulses arrive from the equipment at the office, in the initiation sequence as shown in FIG. 2, the operation of the circuit 15 is shifted from the initial mode to the activation mode (FIG. 5), and then to the communication mode (FIG. 7).
In the communication mode, the circuit 15 executes a call between the terminal equipment and the line termination equipment at the telephone office. When the call is completed, the mode of the circuit 15 is shifted from the communication mode back to the initial mode. Step 600 detects a control signal indicative of loopback test having arrived from receiver 13 during the communication mode, and then step 602 shifts the circuit 15 to the loopback test mode. When the test ends, circuit 15 is returned to the communication mode at step 604.
The loopback test of the NT1 is intended for testing all the functions and performance of the NT1. The conventional system is defective, however, in that it cannot test the call origination signal detector or the call origination transmitter which operates when activated by a call from the terminal equipment as the subscriber line is activated by the equipment at the telephone office at the loopback test. Moreover, it cannot verify the operation of the power feeder which supplies the power to the terminal equipment by such loopback test.
This invention was conceived to overcome such defects encountered in the prior art and aims at providing a NT1 which can test a circuit which operates when the unit is activated by an incoming call from the terminal equipment and/or the power feeder which supplies power to the terminal equipment.